1. Field of the Invention
The invention relates to a speed reducer and an electric power steering system that includes the speed reducer.
2. Description of the Related Art
Generally, in an electric power steering system, torque output from a steering-assist electric motor is amplified by reducing, with the use of a speed reducer, the speed of rotation transmitted from a rotary shaft of the electric motor. The amplified torque is transmitted to a steering mechanism, such as a rack-and-pinion mechanism, whereby torque assist is provided to an operation of the steering mechanism caused in response to a driver's steering operation. As the speed reducer, for example, a worm speed reducer constructed of a worm and a worm wheel that mesh with each other is used. The worm speed reducer includes a housing that houses the worm and the worm wheel, and is supplied with grease.
The grease supplied into the speed reducer lies between tooth surfaces of the worm and tooth surfaces of the worm wheel, and forms oil films. The oil films formed by the grease functions to assist sliding between the tooth surfaces caused by the rotation of the worm due to the rotation of the rotary shaft of the electric motor and the rotation of the worm wheel due to the rotation of the worm, and functions to buffer impacts that occur between the tooth surfaces due to the rotations, thereby reducing rattling noise generated by the impacts. Therefore, supplying the speed reducer with the grease makes it possible to reduce steering torque of the electric power steering system and suppress rattling noise, thereby improving a driver's steering feel.
The characteristics, such as a viscosity, of the grease are appropriately adjusted by taking into account, for example, the use of the grease and the usage environment of the grease. Commonly-used grease has a low fluidity, and therefore is easily extruded from a meshing section at which the worm and the worm wheel mesh with each other, due to a sliding motion of the worm itself caused by the rotations of the worm and the worm wheel. Once the grease is extruded from the meshing section due to the sliding motion, the extruded grease accumulates around the meshing section, and does not automatically return to the meshing section.
Therefore, the meshing section runs out of grease, so that the effect of assisting sliding between tooth surfaces and the effect of buffering impacts between the tooth surfaces, which are produced by the grease oil films, are no longer obtained. Therefore, for example, in the case of an electric power steering system, the steering torque increases and rattling noise is generated. As a result, the driver's steering feel deteriorates. In order to cope with the above-described phenomenon, not only the meshing section but also, for example, substantially the entire space inside the housing is filled with grease.
In some cases, there is provided a structure for circulating the grease extruded from the meshing section to return the grease to the meshing section as the worm and the worm wheel rotate. Refer to, for example, Japanese Patent Application Publication No. 2005-69476 (JP 2005-69476 A), Japanese Patent Application Publication No. 2006-76443 (JP 2006-76443 A), and Japanese Patent Application Publication No. 2007-303487 (JP 2007-303487 A).
In order to fill substantially the entire space inside the housing with grease as described above, a large amount of grease is required. If a large amount of grease is present in the speed reducer, the grease may enter, for example, a rolling bearing and affect the performance of the speed reducer, or the grease may leak through gaps of the housing and make its surroundings dirty. Furthermore, an increase in the amount of grease results in a cost increase.
Even if a large amount of grease is supplied, the grease is extruded from the meshing section with the rotations. After the grease is extruded from the meshing section, the meshing section is not automatically replenished with new grease because the conventional grease has a low fluidity as described above. Therefore, it is not possible to prevent the meshing section from running out of the grease. On other hand, when the mechanism for circulating the grease within, for example, the housing is provided, the grease is automatically returned by the mechanism to the meshing section at which the worm and the worm wheel mesh with each other, as the worm and the worm wheel rotate. Therefore, it is possible to reduce the total amount of grease that is supplied into the housing, and to prevent the meshing section from running out of the grease.
However, provision of the mechanism complicates the structures of the housing, the speed reducer and the like, leading to increases in the costs of the speed reducer, the electric power steering system and the like.